The use of optical methods for testing, measuring, and system operations has become increasingly important. Optical methods are especially important for use in monitoring system operations, particularly when the monitoring of system operations is conducted by means such as absorption, emission, reflectance, fluorescence, or Raman spectroscopy. All of these monitoring methods require that a light beam be guided, usually by an optical fiber, to a detection device or a receptor, which, in many cases, is a spectrograph. In monitoring situations which include the simultaneous analysis of multiple samples or the analysis of points that are spatially separated, either multiple detectors, multiple receptors, or the imaging of multiple inputs on a single detector or receptor is required.
It has been found that prior art approaches for imaging multiple inputs on a single detection device or a single receptor are not always feasible. For example, if one wants to image a two-dimensional area of a sample, as is done in confocal Raman spectroscopy, typically there is only sufficient imaging space available on the detection device for one image. Hence, in order to obtain two-dimensional images from several samples, the light input source must be switched to enable use of a single spectrograph.
The potential applications for a spectroscopic grade fiber optical switch are many. For example, a single spectrograph could be shared among several investigations, reducing the expense associated with duplicating equipment, and conserving precious rack space.
In yet another potential application, a spectroscopic grade fiber optical switch could be used on a planetary lander 100 (FIG. 1) to switch input channels. For example, a single spectrograph could receive inputs from fiber optic probes on the robot arm, fiber optic probes in the bore of a drilled hole, fiber optic probes on lander legs, or fiber optic probes 102 harpooned away from the planetary lander as shown in FIG. 1.
In yet still another potential application (e.g., Raman spectroscopy), the problem exists that inorganic minerals are better analyzed using an incident wavelength in the visible range (e.g., 532 nm), while organic materials are better analyzed in the near infrared wavelength range (e.g., 1064 nm). A fiber optic switch could be used to switch laser light beams to a single, dual wavelength, imaging spectrograph.
Prior art devices exist for routing optical signals, but these prior art devices have many limitations. One prior art method uses a mechanical fiber switch that relies on motors to physically align several optical fibers. These prior art active optical switching methods tend to be slow, bulky, and expensive. Additionally, these prior art active optical switching methods are not suitable for applications in space travel since the moving parts may cold-weld together, thereby disabling the optical switching mechanism.
Another prior art active method for routing optical signals is electro-optical switching. The devices using electro-optical switching have no moving components and provide their switching action by the application of a voltage that produces a phase-shift in a waveguide to redirect the light beam. The electro-optical switching method is fast, but has a wavelength range which is limited to only a few nanometers.
Several prior art passive optical “switching” methods are available. One example of a prior art passive switching method is a polarizing splitter. Polarizing splitters can only redirect a light beam having a specific polarization. However, the use of polarizing splitters results in the loss of information that may be contained in the polarization state of the light. Additionally, using only polarized light results in a 50% loss in intensity.
Another prior art passive switch method is a fused splitter. Fused splitters can also be used to split or combine optical signals between multiple fibers. Specifically, fused splitters are constructed by fusing and tapering two optical fibers together. Fusing and tapering two optical fibers together provides a simple, rugged, and compact method of splitting and combining optical signals. Typical excess losses in fused splitters are low, while splitting ratios are accurate to within ±5 percent at the design wavelength. Fused splitters are bidirectional and offer low backreflection. However, fused splitters suffer from some disadvantages. Specifically, signal intensity in fused splitters is split between the outputs. This splitting between the outputs results in high loss for larger port counts.
Still another type of prior art passive switching method is a multi-mode fused splitter. Multi-mode fused splitters have a somewhat limited spectral range and are mode dependent. Certain modes within one fiber in multi-mode fused splitters are transferred to the second fiber, while other modes are not. As a result, the splitting ratio in multi-mode fused splitters will depend on what modes are excited within the fiber. In comparison, single mode fused splitters only transmit one mode. Accordingly, single mode fused splitters do not suffer from mode dependency. However, single mode fused splitters are even more highly wavelength-dependent. A difference in wavelength of only 10 nm can cause a significant change in the splitting ratio.
Except for some mechanical active optical switching methods which use motors, no prior art switching technique can achieve the broad wavelength range and the low signal loss required for spectroscopic measurement (e.g., Raman spectroscopy applications.) However, optical switching methods that use motors or gears to mechanically align fibers are prone to problems in space (e.g., cold welding, stiction). Therefore, to achieve the full potential of distributed multi-spectral optical sensing, a small, inexpensive, broadband, reliable, fast, and low-loss optical switch is required.
Accordingly, there remains a need in the art for an optical switch suitable for use in fiber-optic spectroscopy which is small, inexpensive, reliable, has no moving parts (causing friction and possible cold-welding in space applications), and is able to cover a large wavelength range.